It is well known that Portland cement has some environmental concerns: the high-energy consumption and the release of air pollutants (NO, and SO2) and greenhouse gases (CO2) related to mining and manufacture.
Significant quantities of fly ash are generated every year. In 2013, the United States produced 115 million tons of coal ashes. While only 45 percent were used beneficially, nearly 64 million tons were disposed of. Based on the analytical CaO content, fly ash can be divided into high-calcium fly ash (CaO content >10%) and low-calcium fly ash (CaO Content <10%). Recent years have seen increasing use of younger lignite or sub-bituminous coal at power plants, which led to increasing availability of high-calcium fly ashes.
ASTM C 618 defines two categories of fly ash: Class F fly ash (FFA) and Class C fly ash (CFA). FFA is produced from the burning anthracite or bituminous coal and contains less than 20% CaO. CFA is produced from the burning of lignite or sub-bituminous coal and contains more than 20% CaO.